Diamond has a number of properties which make it attractive for use as window material, free-standing domes, or other planar and non-planar structures for various applications. Among these properties are extreme hardness and excellent transmissivity of certain radiation. Diamond is also an extraordinary heat conductor, thermally stable, and an electrical insulator. However, natural diamond is prohibitively expensive for applications which require any substantial size and is difficult to form into certain shapes.
In recent years, a number of techniques have been developed for depositing synthetic diamond on surfaces of various shapes to obtain a diamond film or coating on tool surfaces and devices. These techniques include so-called high-pressure high-temperature ("HPHT") methods and chemical vapor deposition ("CVD") methods. The CVD methods include plasma deposition techniques wherein, for example, plasmas of a hydrocarbon and hydrogen are obtained using electrical arcing. The resultant plasma can be focused and accelerated toward a substrate using focusing and accelerating magnets.
In order to obtain diamond films having shapes needed for particular applications, it is desirable to have substrates for diamond deposition that can be readily formed into the appropriate shapes. Graphite is such a material, and synthetic diamond film has been deposited, such as by chemical vapor deposition, on the surface of a graphite substrate. If necessary, the graphite can then be removed, leaving a free-standing diamond film or layer of a desired shape. Graphite can be provided that has a coefficient of thermal expansion that is relatively close to that of diamond film, and in this respect it is favorable for deposition of diamond film. However, diamond film generally does not deposit well on graphite because diamond deposition conditions tend to etch graphite, which leads to erosion of the substrate rather than deposition.
It was previously discovered that deposition of synthetic diamond on a graphite substrate can be improved by providing a thin interlayer of a metal, particularly molybdenum or tungsten. The thin layer of the metal was found to facilitate the adherence of the synthetic diamond being deposited. Although the metal does not match well with the diamond from the standpoint of thermal coefficient of expansion, use of a very thin layer of the metal minimizes the impact of such mismatch.
While coatings or interlayers of metals such as molybdenum or tungsten on graphite have been found to be generally effective, there is room for further improvement.
It is among the objects of the present invention to devise improvements in techniques for deposition of synthetic diamond film on graphite substrates.